1. Field of the Invention
This invention is concerned with a method of recovering oil from a subterranean reservoir. More particularly, this invention relates to an improved oil recovery process in which a slug comprising an aqueous polymer solution containing a small amount of a surfactant injected into the formation via an injection well is forced through the formation and oil is recovered via a production well penetrating the same formation. During this recovery process the concentration of the surfactant in the slug of the aqueous polymer solution is increased during the slug injection time.
2. Description of the Prior Art
The production of petroleum products is usually accomplished by drilling into a hydrocarbon-bearing formation and utilizing one of the well-known recovery methods for the recovery of the hydrocarbons. However, it is recognized that these primary recovery techniques may recover only a minor portion of the petroleum products present in the formation particularly when applied to reservoirs of viscous crudes. Even the use of improved recovery practices involving heating, miscible flooding, water flooding and steam processing may still leave up to 50 percent or more of the original hydrocarbons in place.
Thus, many large reserves of petroleum fluids from which only small recoveries have been realized by present commercial recovery methods, are yet to reach a potential recovery approaching their estimate oil-in-place.
Water flooding is one of the more widely practiced secondary recovery methods. A successful water flood may result in ultimate recovery of up to about 50 percent of the original hydrocarbons in place. However, generally the application of water flooding to many oil reservoirs results in much lower recoveries.
The newer development in recovery methods for heavy crudes is the use of steam injection which has been applied in several modifications, including the "push-pull" technique and through-put methods, and has resulted in significant recoveries in some areas. Crude recovery by this process is enhanced through the beneficial effects of the drastic viscosity reduction that accompanies an increase in temperature. This reduction in viscosity facilitates the production of hydrocarbons since it improves their mobility, i.e., it increases their ability to flow.
However, the application of these secondary recovery techniques to depleted formations may leave major quantities of oil-in-place, since the crude is tightly bound to the sand particles of the formation, that is, the sorptive capacity of the sand for the crude is great. In addition, interfacial tension between the immiscible phases results in entrapping crude in the pores, thereby reducing recovery. Another disadvantage is the tendency of the aqueous drive fluid to finger, since its viscosity is considerably less than that of the crude, thereby reducing the efficiency of the processes.
Consequently, process modifications have been developed which may incorporate additives to lessen the above cited disadvantages and thereby improve the efficiency of these processes. For example, surface-active agents and miscible liquids are utilized to decrease the interfacial tension between the water and the reservoir crude, and thickeners have been developed to adjust viscosity so as to inhibit fingering.
One of the principal objects of this invention is to provide an improved process for increasing the ultimate recovery of hydrocarbon from a hydrocarbon-bearing formation.
Another object of this invention is to provide an efficient method for the recovery of hydrocarbons from a hydrocarbon-bearing formation in which a slug of an aqueous polymer solution containing a small amount of a surfactant is employed and in which the surfactant concentration in the polymer slug is increased as the slug is injected into the formation.